1. Field of the Invention
This invention relates to an electric wheelchair, and more particularly to an electric wheelchair suspension.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional suspension disclosed in U.S. Patent Application Publication No. 2004/0060748 A1 is incorporated in an electric wheelchair, and has two side frame assemblies 1, each of which includes a front caster assembly 11, a rear caster assembly 12, a linkage 13, a drive wheel assembly 14, a driving unit 15 and a return spring unit 16.
The front caster assembly 11 includes a front mainframe 112, a front caster frame 113 extending downwardly from a front end of the front mainframe 112, a drive wheel frame 114 extending downwardly from a rear end of the front mainframe 112, and a front caster 115 disposed pivotally on a lower end of the front caster frame 113.
The rear caster assembly 12 includes a rear mainframe 122, a front connecting frame 123 extending downwardly from a front end of the rear mainframe 122, a rear caster frame 124 extending downwardly from a rear end of the rear mainframe 122, and a rear caster 125 disposed pivotally on a lower end of the rear caster frame 124. The rear mainframe 122 cooperates with the front mainframe 112 to support a seat (not shown).
The linkage 13 has two ends connected respectively and pivotally to the front caster frame 113 and the front connecting frame 123.
The drive wheel assembly 14 includes a drive wheel 141 that has a diameter greater than those of the front and rear casters 115, 125, and that is disposed pivotally on the corresponding drive wheel frame 114.
The driving unit 15 includes an electric motor 151 installed on the corresponding drive wheel frame 114 and operable to rotate the corresponding drive wheel 14.
The return spring unit 16 includes a front spring 162 interconnecting the corresponding front mainframe 112 and the corresponding front connecting frame 123, and a rear spring 162′ interconnecting the corresponding drive wheel frame 114 and the corresponding rear mainframe 122.
Referring to FIGS. 2 and 3, because of the presence of the front and rear springs 162, 162′, the drive wheel 141 and the front and rear casters 115, 125 can be brought into contact with a horizontal surface simultaneously.
Referring to FIG. 2, when the front caster 115 climbs onto an elevated surface 200 in the wheelchair's path, such as a sidewalk area, the front and rear springs 162, 162′ are stretched so as to allow the front caster 115 to rise relative to the drive wheel 141 and the rear caster 125. After the rear caster 125 surmounts the elevated surface 200, the front and rear springs 162, 162′ return to their original shapes so as to allow the front caster 115 to closely contact a top surface of the elevated surface 200.
FIG. 4 shows a time-height curve illustrating the elevated surface surmounting efficiency of the wheelchair installed with the side frame assemblies 1. As indicated by the time-height curve, the wheelchair takes about 4.6 seconds to surmount an elevated surface 200 having a height of about 60 cm. This is a significant amount of time and is indicative of an inferior elevated surface surmounting efficiency.
With such a configuration, when the front caster 115 comes into contact with the elevated surface 200, impact is transmitted directly from the front caster assembly 11 to the rear caster assembly 12 and, thus, to the rider sitting on the electric wheelchair. This results in rider discomfort. Furthermore, the maximum pivoting angle of the front caster assembly 11 is large. This further increases rider discomfort.